1. Field of the Invention
The present invention is directed to DC-DC switched mode power converters utilizing flyback-type converter circuits of the SEPIC (single-ended primary inductor converter) type.
2. The Background
Switched mode DC-DC power converters are common in the electronics industry. They are frequently used to convert one available DC level voltage to another DC level voltage, often needed for a particular set of semiconductor chips. Such power converters generally use one or more electrically controlled switches (such as N- or P-Channel MOSFETs) the gates of which are controlled by a switched mode power supply controller circuit which is often integrated onto a single chip.
As electronic devices become faster, smaller and more portable, the need for increased electrical efficiency in DC-DC converters used in these devices is becoming more important. Energy wasted in portable electronics devices prematurely drains the battery powering the device and creates waste heat which must be managed. Relatively small increases in overall electrical efficiency--such as from 75% to 85%--result in a major decrease in wasted power and waste heat--e.g., from 25% to 15%.
Turning now to FIG. 1, a basic flyback converter circuit adapted as a switched mode power as known in the art is shown. An input DC voltage is applied between the input terminals denoted Vin and GND. GND may be any fixed potential such as 0 volts DC relative to Vin. An input capacitor C1 preferably filters the input and may be disposed between Vin and GND. Winding W1 of inductor L is disposed between Vin and node 10. Switch Q1, which is shown as an N-Channel MOSFET, conducts current from a first terminal (drain) connected to node 10 to a second terminal (source) connected to GND when the gate of Q1 is powered by switching signal SS on line 12 from switched mode controller 14. Switched mode controller 14 may itself be powered from Vin and GND or another convenient power source. SENSE input on line 16 provides an indication of the output voltage Vout to controller 14 so that it may adjust the duty cycle of the switching signal SS to adjust Vout to a pre-programmed output voltage. The pre-programmed output voltage may be set in a number of ways known to those of skill in the art, such as with external components (not shown), built-in components, and the like.
Winding W2 of inductor L is lightly magnetically coupled to winding W1, as in a transformer, and utilizes the turns ratio difference between W1 and W2 to step the voltage approximately to Vout. Additional voltage correction is accomplished by varying the duty cycle on SS as discussed above. Diode D1 is disposed between a first terminal of W2 and Vout. The second terminal of W2 is connected to GND'. Preferably capacitor C2 is disposed between Vout and GND' to act as a filter. GND and GND' may be isolated from one another or connected together, as desired. The polarities of W1 and W2 (as are the polarities of all inductors herein) are as shown by the dots in the schematic diagram.
A disadvantage of this design is the lack of tight coupling between W1 and W2. The energy stored in the leakage inductance is wasted and creates a relatively large voltage spike on Q1 which needs to be clamped. As a result, power is wasted and larger, more voltage tolerant components must be utilized.
It would be desirable to have a high efficiency modification of this circuit to permit the high efficiency conversion of relatively high DC voltages to relatively low DC voltages without the drawbacks of the prior art.